W. MW. Hutchings—Ottrelite in North Cornwall. 215 
The slate is of a greenish-grey colour, close and hard in texture, 
with a silky lustre rather inclining to resinous in the best kinds. 
Moderately thin flakes of it are translucent on the edges. It splits 
easily into very thin sheets, which are strong and sonorous; and 
appears to be a really first-class roofing-material, equal in every way 
to the best products of the celebrated quarries at Old Delabole, a few 
miles inland. 
It is fossiliferous, apparently as much so as the Delabole slate 
so well known in that respect, and Mr. Kinsman informs me that 
the fossils from the two localities are identical in nature. 
There are two quarries at Tintagel very near together, but it is 
only in the slates from the “ New Quarry,” nearest to the church, that 
I have found ottrelite. 
Macroscopically, nothing of its mineralogical composition can be 
made out except that it is more or less speckled over with pyrites, 
and stained here and there with ferric oxide due to the oxidation of 
that mineral. . 
When very thin sections of it are examined under the microscope, 
it is seen that the main constituent is sericitic mica. By far the 
greater part of this sericite lies flat in the plane of cleavage of the 
slate, the flakes overlapping and felted together to an aggregate 
which, between crossed Nicols, forms a dimly-polarizing, blue-grey 
base in which the other minerals are set. A great many flakes 
of sericite, however, are not arranged parallel to the cleavage, but 
lie in all directions, polarizing in brilliant colours. _ 
The two other principal constituents of the rock are ilmenite 
and ottrelite ;—it might be best described as being a sericite-ottrelite- 
ilmenite phyllite. 
The ottrelite is very abundant. It occurs entirely as quite 
irrecular and indefinitely-bounded, mostly more or less ragged flakes, 
dispersed pretty uniformly throughout the rock. 
The arrangement of the flakes bears no definite relationship to the 
plane of cleavage of the slate; they lie flat in this plane, vertical to 
it and at every intermediate degree of inclination. The size is very 
variable, but as a whole it is very much less than in the well-known 
rocks of Ottrez, Serpont, and other places in the Ardennes. The 
largest flakes are usually oblong in shape, the long diameter seldom 
exceeding 4; inch, and reaching this only in the minority of cases. 
A usual average, especially in the more rounded forms, is close 
around +35 inch; but there is ample evidence that the mineral has 
been much torn and broken up since its formation, and lesser flakes 
exist down to exceedingly small fragments. The thickness is as 
variable as the other dimensions. It does not exceed yj5>5 inch, and 
most of the flakes are very much less, down to gy's0 inch, which 
seems to be the thickness of a large proportion of the fairly thin ones 
measured. 
The characteristic secondary cleavages, intersecting on the basal 
plane, are largely developed, though in many flakes they are con- 
siderably distorted, and are also frequently much masked by irregu- 
lar cracks and ruptures, these effects being due to the stress the 
